
Having Him on a lead and having the lead attatched to a sturdy structure helps.(Maybe close the door and have the leash tied round the door handle)
He wont be able to turn round to nip you and you can work near his rear safely, Every time he lets you touch there even a little bit without any fuss give him a small treat (Something really nice that he loves and rarely gets) so he knows he's doing well and he will very quickly learn that this treatment means treats if he just doesnt fuss and when you feel he is comftable with it, you can remove the lead as you know he wont react or bite etc.
We tried this with our friends dog (A Large breed) who would bite if they tried to brush him, it worked wonders and now hes more then happy for a treat and a brush, it also avoids the stress of putting on a muzzle and he will know when the treatment is coming if you out one on him everytime!
Let us know how you get on!